Semiconductor nanoparticles have a feature of being able to control an energy level variously according to size, shape, and composition thereof, and have good electric conductivity and great absorbing cross-sectional areas in comparison to organic matters. Therefore, various researches to apply such semiconductor nanoparticles to a high efficiency solar cell are ongoing.
To implement the solar cell using the semiconductor nanoparticles, processes of removing non-conductive surfactant from surfaces of the nanoparticles and implementing the nanoparticles in a film state are essential. In particular, when the solar cell is manufactured using a hybrid of semiconductor nanoparticles and conducting polymer, the amount of surfactant remaining on the surfaces of the nanoparticles greatly affects characteristics of the solar cell.
FIG. 1 illustrates an example of a related-art method of manufacturing a solar cell using nanoparticles. As shown in FIG. 1, a hybrid light-absorbing layer is manufactured by mixing surface-treated semiconductor nanoparticles and organic semiconductor material in a conducting polymer solution, and applying the mixture. By treating the surfaces of the nanoparticles which have been synthesized using amine-based material (for example, pyridine, 1-butylamine, etc.), ligands are removed from the surfaces, and a nanoparticles solution based on a non-polar solvent such as chloroform (CHCl3) is manufactured. After that, a solution for manufacturing a light-absorbing layer is made by mixing the nanoparticles solution and the conducting polymer solution at a predetermined ratio, and is applied to a transparent electrode (spin coating, etc.) and is made as a film.
However, such a related-art method has a problem that colloidal stability of the surface-treated semiconductor nanoparticles is low. That is, because there is no surface ligand in the solution of the surface-treated nanoparticles or the ligand has a short length, the nanoparticles are not stabilized and precipitation easily occurs in the light-absorbing layer solution. This undermines long-term colloidal stability and also degrades reproducibility because deviation is great when devices are manufactured. Furthermore, reliability may deteriorate when the solar cell is scaled up afterward.
FIG. 2 illustrates another related-art method of manufacturing a solar cell using nanoparticles. This method is a layer-by-layer assembly method that manufactures a nanoparticles solar cell by stacking nanoparticles of a single layer several times or dozens of times. This method uses a bifunctional linker when stacking the nanoparticles layer repeatedly so that the semiconductor nanoparticles can be prevented from being washed away, and also, reduces a distance between the nanoparticles so that charge transfer between the nanoparticles can be improved.
This method has the merit of maintaining colloidal stability of the nanoparticles, but has problems that it takes much time to manufacture a device since the stacking/washing process should be performed dozens of times in order to manufacture a light-absorbing layer to absorb light sufficiently, and that device characteristics are greatly changed according to a kind of bifunctional linker.